Wednesday, August 24, 2011

Nitrogen Pressure and Spectral Resolution

Super-conducting NMR magnets are cooled with liquid helium. The boil off rate of the liquid helium is minimized by surrounding it with a high vacuum and a liquid nitrogen cryostat. The liquid nitrogen cryostat may either be vented directly to the atmosphere or it may be maintained at a pressure slightly above atmospheric pressure with the use of pressure relief valves. NMR users should be aware that the homogeneity of an NMR magnet is affected drastically by the pressure in the liquid nitrogen cryostat. The left-hand panel of the figure below shows a well resolved 400 MHz 1H NMR spectrum of the methyl triplet of ethylbenzene in a well shimmed magnet where the nitrogen cryostat was maintained slightly above atmospheric pressure with pressure relief valves for several days. The spectrum in the right-hand panel was run under identical conditions after the cap to the nitrogen fill port was removed, relieving the pressure in the nitrogen cryostat.

3 comments:

Anonymous
said...

Hi Glen,

Did you reshim the magnet after removing the pressure cap? The difference that you see is because the nitrogen dewar being under a lower pressure has a slightly different geometry (a few microns) than being under a higher pressure. You should normally be able to obtain the same lineshape after some shimming.In fact what you refer to as a pressure release valve is only a safety valve. Nitrogen evaporates through the other ports of the dewar and this valve is there just to prevent the dewar overpressurising and exploding if the other ports are blocked. There is no nitrogen flow through this valve under normal operation, if there was you should have seen some condensation or ice on it. The pressure released when you remove it is actually the back pressure from the tubing at the other ports and the flow meter, if present. Quite low pressure but at a big dewar thus a lot of gas released. Also the low pressure causes a very slight geometrical deformation on the cryostat (a few micrometer so) but one which is clearly visible if you observe ppm changes.

Thank you for the very detailed explanation. I am aware that the cap in the photograph is for safety purposes in the event of over pressurization. The pressure relief valve I am referring to in the BLOG post is the one-way valve through which the nitrogen gas normally vents and maintains a positive pressure inside the nitrogen vessel. I photographed the safety valve only to show the nitrogen vessel closed under positive pressure with a well shimmed spectrum compared to an open nitrogen vessel under atmospheric pressure and a spectrum with the same shims. See also this post ( http://u-of-o-nmr-facility.blogspot.ca/2011/09/resolution-during-liquid-nitrogen.html ). It is really remarkable that such a small change in pressure can make such a difference. Thank you again for this comment.

OK, now it kind of makes more sense. The one way valve is there not to maintain a higher pressure in the nitrogen can but to ensure that there will be no accidental reverse flow and condensation of liquid air in the nitrogen can. This could happen in theory if a very high pressure system sets above and there is not enough nitrogen evaporating for a period of time. These one-way valves have a very small spring and thus create a bit of back pressure. They are not there to maintain the pressure at a specific level.Regarding your other post the changes you see are because you add weight to the nitrogen can in the form of liquid nitrogen. This weight makes the can pull on the helium turrets a bit harder causing a bit of deformation, again a few microns (don't forget that the whole of the magnet hangs from the helium turrets!). This is again enough to alter the lineshape.All these changes in geometry can be compensated with shims, so if you want to be fair you can remove one nitrogen port cover or stop the refill half-way. You should be able to shim the system back to lineshape. Just remember to put the covers on again, otherwise you will have nice big ice blobs next morning.In NMR we observe differences of 0.1 or 0.01 of a ppm. As such it is quite remarkable what things you can see. You must have seen people observing perturbations because of trams kilometres away and I remember once seeing the effect of a door opening and closing in the lab during a stability test. It should be no surprise that lineshape is affected if you do something on the magnet itself!